Pulse-echo acoustic ranging systems, also known as time-of-flight ranging systems, are commonly used in level measurement applications. Pulse-echo acoustic ranging systems determine the distance to a reflector (i.e. reflective surface) by measuring how long after transmission of a burst of energy pulses the echoes or reflected pulses are received. Such systems typically use ultrasonic pulses or pulsed radar or microwave signals.
Pulse-echo acoustic ranging systems generally include a transducer and a signal processor. The transducer serves the dual role of transmitting the energy pulses and receiving the reflected energy pulses or echoes. An echo profile is generated from the received energy pulses. Echo pulses are identified in the echo profile by the signal processor, and the distance or range of the object is calculated based on the transmit times of the transmitted energy pulses and the receive echo pulses.
The commonly used technique for finding echoes in an echo profile involves generating a time varying threshold or TVT curve. The TVT curve provides a baseline or line on the echo profile which is above the noise level in the echo profile. Valid echoes appear above the TVT curve. Various algorithms and techniques are known in the art for the generating the TVT curve.
A typical echo profile indicated by reference 100 is shown in FIG. 1 together with a TVT curve indicated by reference 120. The first portion of the echo profile 100 comprises a half pulse 140 which corresponds to the ring down in the transducer. The ring down corresponds to the period in which the transducer is still ringing down from the transmit pulses emitted and as such it is very difficult to detect reflected energy pulses. As shown in FIG. 1, the ring down period 140 falls underneath the TVT curve 120 and is treated as noise. Following the ring down 140, the echo profile 100 comprises a number of pulses 160, indicated individually as 160a, 160b, 160c, 160d, 160e and 16f, in FIG. 1. Using the TVT curve 12, the pulses 16a, 16b, 16c, 16d and 160e are identified as valid receive echo pulses. The last pulse 160f falls below the TVT curve 120 and is considered to comprise noise.
While the TVT technique has been used successfully in level measurement and time-of-flight ranging systems, there are shortcomings. First, generating the TVT curve can be a processor intensive process. Secondly, most TVT curves require manual adjustments to provide the best performance, and different TVT curves will work better in some situations than others. Thirdly, echoes in the ring down portion cannot be identified using a TVT curve.
Accordingly, there remains a need to provide a system and techniques which improve the processing of the reflected energy pulses or echoes.